The primary objective of the proposed study is to assess the impact and cost effectiveness of interventions aimed at improving adherence within a clinical trial, in this case a trial of oral potassium supplement to antihypertensive therapy. From a specialty hypertension clinic at the University of Massachusetts Medical Center, 180 patients with well established drug treatment of essential hypertension will be recruited. Patients will be excluded only on the basis of renal impairment, hyperkalemia, need for potassium-sparing diuretics, other unstable concurrent disease, or reduced dietary sodium intake as determined by 24-hour urinary sodium levels below 120 mmol. In a parallel group design study, subjects will be randomly assigned to receive 60 mmol of an oral potassium preparation in three divided doses per day, or a matching placebo. Within these double-blinded treatment groups, patients will additionally be randomized to one of three adherence programs while maintaining stable antihypertensive drug treatment. The three adherence intervention programs will consist of (1) standard care, (2) telephone contact once every three weeks to encourage adherence, and (3) a 30 minute program of education and individual counseling with follow up counseling four weeks later. These patient-centered, educational/ behavioral counseling sessions will be based on knowledge of patients' health beliefs and other variables linked to likelihood of adherence, obtained from questionnaires administered at the initiation of the study. During twelve weeks, the influence of a) oral potassium upon sitting systolic and diastolic blood pressure, the adherence interventions upon serum potassium concentration, and urinary sodium and potassium excretion over 24 hours, and b) adherence interventions on specific adherence measures, will be determined. The primary method of assessing adherence will be percentage of observed medication vial openings in relation to the number anticipated, recorded using the medication event monitoring system (MEMS). An additional major adherence assessment in those patients assigned to receive oral potassium will be the extent of increase in daily urinary potassium excretion. Secondary measures of treatment adherence will include tablet counts, and patient interview. The different measures of adherence will be compared to assess which correlate with one another. The proposed project will be a collaborative effort between researchers in the Graduate School of Nursing and the Department of Medicine of the University of Massachusetts Medical Center.